Air - Clear quartz, or any crystal bead (lead crystal, rock crystal, etc.), or any white or yellowish stone.

Fire - Haematite, flame Agate, or any red/orange stone.

Water - Lapis Lazuli, blue Amethyst, any other blue stone.

For Spirit you can also use clear, white, or purple stones (Amethyst seems to be the preferred one). The reason why it's so difficult to choose a stone for Spirit is because Spirit encompasses all the elements within it, and most stones are associated with one or two elements. You could instead use a "birth stone" for Spirit--one associated with your birth month and/or zodiac sun sign.

For more info on this, check out the books by Scott Cunningham or Gerina Dunwich.